Vaccines have traditionally used complex immunogens such as inactivated viruses or bacteria to induce an immune response. Such vaccines were often associated with adverse side effects including hypersensitivity and the risk associated with incomplete inactivation. The use of subunit vaccines, such as antigen-based vaccines, has reduced the number and severity of unwanted side effects associated with vaccines produced with more complex immunogens. Subunit vaccines are generally composed of only one, or a few, proteins or polysaccharides from the target pathogen. Some vaccines may have a larger number of proteins or polysaccharides to provide immunity against a large number of related serotypes. Due to their small size and limited number of epitopes, by themselves, subunit vaccines tend to be only weakly immunogenic, often failing to induce a satisfactory immune response. To be effective, the use of subunit vaccines requires strategies to enhance immunogenicity.
One means to enhance vaccine immunogenicity is through the use of specific adjuvants. Immunological adjuvants are the component(s) of a vaccine which augment the immune response to the antigen. Precipitated aluminum salts, generically referred to, in the singular, in the field of vaccine adjuvants as an aluminum adjuvant or “alum”, are the most widely used adjuvant in human vaccines. Not wishing to be bound by theory, several theories have been proposed for how aluminum adjuvants stimulate the immune system. One such theory is that aluminum adjuvant provides a depot of antigen at the site of administration, thereby providing a gradual and continuous release of antigen to stimulate antibody production. Aluminum adjuvants may also function by inducing a mild inflammation reaction at the injection site which primarily stimulates IL-4 and T-helper-2 cells that enhances IgG1 and IgE production. One common aluminum adjuvant is aluminum hydroxide (AlO(OH)) which is Al+3 precipitated with OH−. Another common aluminum adjuvant is aluminum hydroxyphosphate which is formed by precipitating Al+3 with PO4 and OH−. Aluminum hydroxyphosphate (Al(OH)x(PO4)y) does not have a fixed stoichiometry. The P/Al molar ratio in the aluminum adjuvant can range from just above 0 (similar to aluminum hydroxide) to approximately 1 (similar to aluminum phosphate). The extent and strength of antigen binding to the aluminum adjuvant is influenced by the properties of the aluminum adjuvant, particularly the chemical composition which is typically defined by the P/Al molar ratio along with the surface charge and size of the primary aluminum adjuvant particle.
Aluminum hydroxyphosphate is most commonly prepared by a batch precipitation method with three reactants: aluminum chloride (or other source of aluminum such as potassium aluminum sulfate), sodium phosphate and a base such as sodium hydroxide. During the course of a batch precipitation, the composition of the reacting mixture can change dramatically, leading to the production of adjuvant that is somewhat different from the start of the batch to the end of the batch. The result can be a heterogeneous mixture with some kind of “average” of properties. See Klein et al., 2000, J. Pharmaceutical Sciences 89:311-321. Not surprisingly, among commercially available aluminum phosphates from different suppliers, properties can vary significantly. For example, aluminum hydroxyphosphates can have molar ratios of phosphate to aluminum in the precipitated solids from significantly less than 0.9 to greater than 1. Others demonstrate poor particle size uniformity. These variations can present consistency problems for the commercial sale of vaccines.
Fed-batch precipitation at a constant pH improves this situation, but requires an active pH control feedback loop. See Burrell et al., 2001, Vaccine 19:275-281 and Burrell et al., 2001, Vaccine 19:282-287.
What is needed are more robust and reproducible methods for the manufacture of aluminum phosphate for use as an adjuvant.